The present invention relates to an apparatus for measuring seal defects on the seal surface of a capture feature tang of a rocket motor case field joint. In particular, the present invention can be used to measure the depth of defects (or repairs to such defects) in the seal surface of a rocket motor casing caused by the firing of the rocket motor.
Rocket motor casings can be refurbished and reused after firing. The seal surface of a rocket motor after firing, however, typically becomes pitted and/or scarred. Thus, before the rocket motor casing can be reused, these defects must be repaired by smoothing out, or blending, the defects into the surrounding seal surface. The depth of these "blend zones" must be less than a specified engineering maximum to ensure proper sealing of the rocket motor case field joint.
More particularly, a case field joint of a rocket motor includes an annular clevis joint and a mating annular tang joint. The interface between the clevis joint and the tang joint must be sealed against the internal pressure generated inside the motor casing when the rocket motor is fired. As shown in FIG. 1, to accomplish this sealing, the tang joint 10 includes a capture feature 12 spaced from a tang 14. The capture feature 12 is sealed against the clevis joint with a capture feature O-ring. The tang 14 includes a tang seal surface 16 upon which primary and secondary clevis joint O-rings are seated to completely seal the clevis joint to the tang joint.
A capture feature gap 18 is defined as the space between the capture feature 12 and the tang 14. The width of the capture feature gap 18 is measured from the interference fit diameter surface 20 (the interior surface of the capture feature 12) to the seal surface 16. The interference fit diameter surface 20 is the engineering datum from which the capture feature gap 18 is measured and is typically unaffected by rocket firing.
To ensure proper sealing between the clevis joint and the tang joint, the capture feature gap 18 cannot exceed a specified engineering maximum. As discussed above, rocket firing can scar the tang seal surface, thereby effectively increasing the depth of the capture feature gap 18.
Before the rocket motor can be reused, the tang seal surface 16 must be refurbished. In that regard, the scars and/or pitting in the seal surface are blended out with a grinding tool into a blend zone 22. Every defect in the tang seal surface 16 must be blended out to an area 30 times the depth of the defect. It would not be unusual for the tang seal surface 16 to have hundreds of blend zones, including overlapping blend zones, after refurbishment.
After refurbishment and before the rocket motor casing can be returned to service, the depth of the blend zones must be measured to determine whether the capture feature gap 18 including the depth of the blend zone 22 exceeds the engineering maximum.
Heretofore, the depth of blend zones was measured using a bore micrometer. In this process, the bore micrometer was first calibrated against a known bore of a ring gauge. Once calibrated, the capture feature gap 18 was measured with the bore gauge at an area of the tang seal surface 16 unaffected by blend zones. With that measurement, the unaffected capture feature gap 18 was determined by adding the bore gauge measurement from the known diameter of the ring gauge.
Once the depth of an unaffected capture feature gap 18 was determined, the depth of the gap at a blend zone was determined by using a "horseshoe gauge." The "horseshoe gauge" comprises a known dial indicator disposed on a C-shaped bracket. The "horseshoe gauge" was first zeroed-out at an unaffected area on the tang seal surface adjacent to a blend zone. Once zeroed, the "horseshoe gauge" was moved to the blend zone to determine the depth of the blend zone. The overall capture feature gap was determined by adding the unaffected capture feature gap to the depth of the blend zone.
The foregoing technique presented several problems. First, the accuracy of this procedure depended upon the operator keeping the bore micrometer and "horseshoe gauge" square to the tang seal surface. If measurements were made at an angle, errors were introduced into the measurement.
Second, this procedure depends upon finding an unaffected area on the tang seal surface. As stated above, however, the tang seal-surface can be covered with hundreds of blend zones, making it extremely difficult to accurately identify an unaffected area of the tang seal surface.